1. Field of the Invention
The present invention relates to the manufacturing method of semiconductor device, in particular, present invention relates to a light emitting device comprising an organic light emitting device (OLED) formed over a substrate having an insulating surface. The invention also relates to an OLED module in which ICs including a controller, or the like, is mounted with an OLED panel. Note that, in this specification, the light emitting device includes the OLED panel and for the OLED module. Electronic equipment using the light emitting device is also included in the present invention.
Noted that in the present specification, the term “semiconductor device” generally indicates a device which is capable of functioning by utilizing semiconductor characteristics, and a light emitting device, an electro-optical device, a semiconductor circuit and an electronic device are all included in the semiconductor device.
2. Description of the Related Art
Recently, technology for forming TFTs (Thin Film Transistor) over a substrate has been greatly progressed, and its application to an active matrix display device is actively developed. In particular, a TFT using a polysilicon film have a higher field effect mobility (also referred to as mobility) than that of a conventional TFT using an amorphous silicon film, and thus, is capable of high-speed operations. Therefore, a driving circuits that consist of TFTs using a polysilicon film is provided over the same substrate as pixels, and the development for controlling respective pixels is performed actively. Since driving circuits and pixels over one substrate are incorporated into an active matrix display device, there are various advantages such as reduction in the manufacturing cost, miniaturization of the display device, improvement in yield, and improvement in throughput.
In addition, an active matrix light emitting device (hereinafter, simply referred to as light emitting device), which has as a self-luminous element an OLED is actively researched. The light emitting device is also referred to as organic EL displays (OELDs) or organic light emitting diodes (OLEDs).
An OLED is self-luminous to have high visibility, and is optimal for making a display thin since a backlight like used for a liquid crystal display (LCD) is not required. Further, an angle of view has no limits. Therefore, a light emitting device using an OLED has thus come under the spotlight as a substitute display device for CRTs and LCDs.
An active matrix driving system for displaying an image by arranging a plurality of TFTs in each pixel and sequentially writing a video signal is known as one mode of a light emitting device using OLED elements. The TFT is an indispensable element for realizing the active matrix driving system.
In addition, for the purpose of realizing the active matrix driving system, in the light emitting device using OLED, since TFT controls current amount flowing through OLED, it can not realized when TFT that uses low current effect mobility amorphous silicon is adopted. It is preferable that the semiconductor film having a crystallizing structure, typically, TFT using polysilicon is adopted to connect to OLED.
The semiconductor film having crystalline structure, typically, a polysilicon film is used to form TFT, and pixels and driving circuits are formed integrally over the same substrate, thereby the number of connecting terminals is dramatically reduced and a frame area (the periphery portion of the pixel portion) is also reduced.
However, even when the TFT is formed by using the polysilicon, its electrical characteristics are finally not equivalent to the characteristics of a MOS transistor formed in a single crystalline silicon substrate. For example, the electric field effect mobility of a conventional TFT is equal to or smaller than 1/10 in comparison with the single crystalline silicon. Further, the TFT using polysilicon has a problem that is dispersion is caused easily in its characteristics due to a defect formed in a boundary of a crystal grain.
In the light emitting device, at least a TFT functioning as a switching element and a TFT for supplying an electric current to an OLED are generally arranged in each pixel. A low off-electric current (Ioff) is required in the TFT functioning as the switching element while high driving ability (an on-electric current Ion), the prevention of deterioration due to a hot carrier effect and the improvement of reliability are required in the TFT for supplying the electric current to the OLED. Further, high driving ability (the on-electric current Ion), the prevention of deterioration due to the hot carrier effect and the improvement of reliability are also required in the TFT of the data line driving circuit.
Moreover, since the luminance of a pixel is determined by the ON current (Ion) of TFT which is electrically connected with an OLED and supplies current to the EL element without depending on the drive method, there is a problem dispersion is caused in luminance if ON current is not constant in case of displaying white on overall surface. For example, in case of adjusting luminance by light emitting time and performing 64 gray scales, the ON current of the TFT which is electrically connected with the EL element and supplies current to the OLED is dispersed 1.56% (=1/64) from a fiducial point to shift one gray scale.
Moreover, when OLED is formed, the gap of EL layer pattering and unevenness of the thickness of EL layer disorder the substrate. There is a slightly variation in luminosity. This invention makes it the subject to be made in view of the above-mentioned problem, to reduce the characteristic variation of each TFT, and to reduce the variation in luminosity.
Moreover, it is also making into the subject to reduce the variation in OLED which is not related to the characteristic variation of TFT, and to reduce the variation in luminosity.